The invention also relates to a device for carrying out the process.
U.S. Pat. No. 2,070,263 describes a method for obtaining aqueous solutions of hydrobromic acid which consists, in a first step, in passing hydrogen through liquid bromine maintained at a temperature of between 37.degree. C. and 42.degree. C. in order to form a mixture of bromine and hydrogen gas which is burnt at a temperature of between 600.degree. C. and 850.degree. C. By working in this way, it is difficult to have an intimate mixture of bromine and hydrogen in stoichiometric amounts on account of the difficulties in rigorously maintaining the temperatures and the thermodynamic equilibria.
An instability of the combustion flame has also been observed, in devices based on the direct combustion of bromine in hydrogen according to an H.sub.2 /Br.sub.2 molar ratio of greater than 1, and for which no technique for mixing the reactants is mentioned, this instability being manifested in particular by strong vacillation of the flame at the burner outlet, going as far as a detachment of the flame ("blow off") from the said burner, which may entail a risk of explosion and a fluctuating quality of the hydrogen bromide gas produced.
Furthermore, such flames become extended forming cones at the base of which are regions from which the reactants are liable to escape without being burnt.
This disrupts the combustion of the bromine in the hydrogen and results especially in residual bromine in the combustion gases, this being of a nature to bring about a considerable decrease in the lifetime of the burners, limit the range of materials which may be used and degrade the quality of the hydrogen bromide gas, thereby preventing it from being used as a reactant for downstream syntheses (secondary reactions, colorations of the products) or for the preparation of hydrobromic acid solutions.
Patent FR 2,365,516 proposes a process which improves the stability of the flame resulting from the combustion of bromine in hydrogen by establishing a helical stream of bromine in a cylindrical chamber, then injecting the hydrogen radially towards the outside in the helical stream of bromine and continuously supplying a flame close to the chamber with the helical stream of bromine and hydrogen.
This process, using a molar excess of hydrogen of 2.6%, leads to an HBr gas containing 300 ppm of bromine by volume, which still gives rise to colorations of the downstream synthesis products as well as the drawbacks mentioned above.
In addition, the complexity of the burner entails a lack of flexibility. Thus, in particular, when it is desired to increase the capacity of the said device, several burners are arranged side-by-side in the same chamber. In such an arrangement, it cannot be avoided that the flames from different burners mounted in parallel will interfere with each other, and furthermore this arrangement is unacceptable with regard to obtaining good distribution of the reactants. This configuration inevitably leads to a lowering in the degree of conversion of the bromine, complicates the control of the cooling of the HBr formed and increases the risks of explosion.
A process, the subject of the claims of the parent application, has now been found for the preparation of pure hydrogen bromide gas by direct combustion of bromine in hydrogen according to the reaction H.sub.2 +Br.sub.2.fwdarw.2HBr, characterized in that it consists in carrying out the following steps successively:
intimately mixing hydrogen and an oxidant in a chamber under a pressure greater than or equal to atmospheric pressure, PA1 starting a flame at the outlet of the said chamber, PA1 replacing all or part of the oxidant by a stream of prevapourized bromine, in an H.sub.2 /Br.sub.2 molar ratio which is sufficient to maintain a stable flame at the outlet of the said chamber and in a so-called combustion zone, thereby making it possible to obtain complete combustion of the bromine in the hydrogen, PA1 cooling the combustion gases in a cooling zone, then PA1 recovering a flow of hydrogen bromide gas under a pressure at least equal to 1 bar absolute and preferably under a pressure of between 1.3 bar absolute and 10 bar absolute and at a temperature not above 125.degree. C. and preferably between 40.degree. C. and 125.degree. C.
According to the process, any mixture consisting of a gas which is inert towards the reacrants of the reaction for the formation of HBr and an amount of oxygen which is sufficient to lead to a combustible sure with hydrogen may be used as oxidant. Nitrogen may be used as inert gas. The oxidant is preferably air.
According to the process, the oxidant may be partially or totally replaced. In the case where a certain amount of oxidant is maintained, this amount may vary within a wide range which depends in particular on the organic impurities present in the supply stream of bromine gas, the size characteristics of the apparatus and the use downstream of The HBr produced.
Hydrogen and the oxidant are introduced in gaseous form into the chamber at room temperature. When the flame is started, the oxidant is replaced by bromine gas introduced at a temperature slightly above its boiling point.
The pressure in the chamber is at least equal to 1 bar absolute, preferably greater than 1.3 bar absolute and even more preferably between 1.8 bar absolute and 10 bar absolute.
It would not constitute a departure from the invention if the pressure was slightly below atmospheric pressure.
The H.sub.2 /Br.sub.2 molar ratio is greater than 1. Preferably, the H.sub.2 /Br.sub.2 molar ratio is greater than 1 and less than 1.4. It has been observed that, under the reaction conditions of the process, the bromine is consumed instantaneously and almost completely.
The reaction of bromine and hydrogen is accompanied by a release of heat (12.3 kcalories per mole of HBr gas formed) which raises the temperature of the combustion flame to more than 1600.degree. C. under adiabatic conditions.
According to the process, the cooling of the hydrogen bromide formed starts from the moment of its formation in the combustion zone and then continues in a cooling zone designed such that the temperature falls gradually in these zones and, at the outlet of the cooling zone, is in the region of 40.degree. C. and 125.degree. C. The pressure prevailing in the said cooling zone is at least equal to 1 bar absolute and is preferably between 1.3 bar absolute and 10 bar absolute.